Previous studies have established the clinical application of acid phosphatase isoenzyme 5 for the diagnosis of hairy cell leukemia. Recent study has clarified the relationship of isoenzyme 5 to prostatic acid phosphatase and other acid phosphatases. New clinical applications of acid phosphatase isoenzyme 5 has been recognized recently. The new proposal includes a continuation of our study to characterize the biochemical properties of isoenzyme 5 of hairy cell and an expansion of our present study to the enzymes in pathologic serum and in normal mouse tissues. Special emphasis now is placed on its application as a marker of osteoclasts and Gaucher disease.